It's not closed source--the encryption library used is OpenPGPjs, which has been extensively audited, and the client, which is where all the encryption and decryption happens, is also open source: https://github.com/ProtonMail/...
Nobody can really guarantee against back doors, but using open source certainly helps, as the more eyes on it the better.
Also, not that it's terribly relevant, but there are at least 4 PhDs working for ProtonMail.
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It's not closed source--the encryption library used is OpenPGPjs, which has been extensively audited, and the client, which is where all the encryption and decryption happens, is also open source: https://github.com/ProtonMail/... Nobody can really guarantee against back doors, but using open source certainly helps, as the more eyes on it the better. Also, not that it's terribly relevant, but there are at least 4 PhDs working for ProtonMail.